1. The present study was undertaken to determine the influence of obesity on bile acid kinetics and cholesterol balance in man. 2. Fourteen obese and normolipidaemic patients (160 +/- 6% of ideal body weight, mean +/- SEM) were studied under standardized dietary conditions. Bile acid kinetics, were determined with the aid of 14C-labelled cholic acid and chenodeoxycholic acid. Cholesterol balance was calculated as the sum of bile acid synthesis plus daily faecal excretion of neutral C27 steroids minus dietary intake of cholesterol. The results obtained were compared with previously published data on control subjects (n = 13). 3. The cholesterol balance was higher in the obese patients (2.61 +/- 0.27 mmol/day) than in the control subjects (1.78 +/- 0.22 mmol/day), owing to a higher excretion of neutral steroids. When expressed per kg of body weight the cholesterol balance was quite normal in the obese patients.